Melting Points

Introduction

A meltemp apparatus is a specific piece of chemistry equipment that can heat samples at a constant rate. When used in conjunction with an accurate digital thermometer one can witness a crystalline solid melt into a liquid and determine at what temperature the change took place. With this information, conclusions can be drawn on the substance that was tested in terms of the purity of a given compound or in identifying an unknown substance. Can you be a bit more specific about these conclusions? How can a melting point help you understand the relative purity of a sample?


Procedure



1. Measure the melting point of one of these compounds, as assigned: 1-indanone or 3,4-dimethoxybenzaldehyde. Repeat the melting point determination 3 times, each time allowing a different person in your group to watch the melting occur. Record your results on the chalkboard at the front of the room.
2. Weigh out 0.20 g 1-indanone and 0.25 g 3,4-dimethoxybenzaldehyde. Take the chemicals back to your working area for later use.
3. Place the two substances in the bottom of a small beaker. Crush and mix them thoroughly, using a glass stir rod or the back of a spatula. Complete mixing may take several minutes. Record any relevant observations. What do you observe? What do you think has happened? What has happened to the melting point?
4. Add approximately 0.05 g (which is just about the size of the head of a pin) of powdered NaOH to the beaker and mix thoroughly for about 5 minutes. Allow the mixture to stand for an additional 10 minutes.
5. Add about 2 mL of 10% HCl to the beaker. Test the pH of the resultant mixture by dipping the end of a glass rod into the mix and then touching it to a piece of pH paper. If the solution is not acidic, add 1 mL HCl and test again. Collect the solid product by vacuum filtration, using the aspirator to draw liquid through the filter.
6.Dry the solid in a drying oven for 15 minutes, then test the melting point of the dry substance.

For more information refer to Melting Points



Part 1-
Data

1-indanone, melting point results are:

Trail
Sweating to Melting Points
1st Trail
39.4°C - 44.6°C
2nd Trail
40.7°C - 43.1°C
3rd Trail
39.8°C - 42.6°C


3,4-dimethoxybenzaldehyde melting point (the data was required from other groups in lab)

Groups
Sweating to Melting Point
1st Group
43.8°C - 44.7°C
2nd Group
41.9°C - 45.2°C
3rd Group
42.3°C - 45.5°C

Observations Thanks for including these! General observations are important and easy to ignore!
In this procedure the 1-indadone was presented as a white crystalline powder, with no noticeable scent. The 3,4 dimethoxybenzaldehyde was a opaque brown crystalline powder also with no noticeable scent.
After 0.20 g 1-indanone and 0.25 g 3, 4-dimethoxybenzaldehyde was mixed in a 150 ml beaker. The two substances when mixed began to sweat and liquify. The mixture of 0.20 g 1-indanone and 0.25 g 3, 4-dimethoxybenzaldehyde was a liquid at room temperature. The melting point of the 1-indanone and 3, 4-dimethoxybenzaldehyde mixture is much lower than the melting points of the pure substances. The completely melted mixture appeared brownish/yellow liquid. After adding a pen tip size of NaOH, the mixture solidified and yielded a yellowish-green color. After about 5 minutes, the liquid mixture turned into a thick honey substance. In about 10 minutes, the mixture completely hardened.
After 2 ml of 10% HCl was added, no reaction was observed.The pH test indicated the mixture was pH 1. This result showed that the product was acidic. After the substance was placed in the oven for roughly ten minutes the powder was packed into a capillary tube and the melting point apparatus measured melting points at 154 degrees Celsius(sweating) -178.8 degrees Celsius(liquified).
Exactly how much of each substance did you weigh out? Record the masses to the nearest 0.001 g.

Part 2- Urea

PureUrea
Sweating
Melting

134
°C
136 °C
Mixture 1:1
105 °C
107 °C
Does the thermometer only give you the temperature to the nearest degree? I thought we could get to 0.1 degree C. If so, record the temps with that level of precision.


Analysis

The compounds in this lab underwent a few physical changes as they were combined. The result was two molecules that had similar melting points before being combined and a dramatically higher melting point when combined. Lower m.p. before they reacted. The product of the reaction had a higher m.p.. The end result was a mixture of two substances will yield a different melting point than the two initial substances.
I'm a bit afraid that you have a misunderstanding of what happened here.

Potential sources of error?


Post lab question


0.3g unknown substance provided
Melting point of substance A: 83.0 deg. C
Melting point of substance B: 83.1 deg. C
Which substance is the unknown? How could it be identified?




By testing the melting points of all the given substances a comparison of results to other substances melting points to determine the unknown. The substance that is known that you would be testing the melting point of, would be consistently melting at the same temp of the unknown substance(83.0 deg C). The level of precision necessary would be difficult to achieve. There is another way! Look up "mixed melting point" on the web and I think you can figure out how it works.